1. Field of the Invention
This invention relates to an electrophotographic type image forming apparatus such as a laser printer, a copying machine, a facsimile machine and the like. More particularly, it relates to an image forming apparatus for charging the surface of a photosensitive body by means of physical contact of a charging member to the rotating photosensitive body.
2. Description of the Prior Art
Conventionally, there is known an electrophotographic type image forming apparatus as represented, for example, by a copying machine, in which an image (or picture) is formed through a sequence of processes which will be described hereinafter.
The surface of a drum-like or belt-like photosensitive body is uniformly charged by a charger and is then exposed by an exposure device so that an electrostatic latent image is formed thereon. Subsequently, a toner adheres to the latent image on the surface of the photosensitive body by a developing roller disposed in a developing device so that the latent image is visualized. The image-visualized toner is then transferred to the surface of a transfer paper supplied from a paper supplying portion or unit, by a transfer device. The toner transferred to the surface of the transfer paper is then fixed by a fixing device. Thereafter, the transfer paper is discharged. Finally, toner remaining, if any, on the surface of the photosensitive body is removed by a cleaning device.
An image forming apparatus of this type employs a corona discharge system as a means for uniformly charging the surface of the photosensitive body. According to this corona discharge system, the surface of a photosensitive body is charged by ionizing a discharging space.
However, in the corona discharge system, when the surface of the photosensitive body is charged, a large amount of ozone is produced. The ozone has the characteristic that when a minus discharge is made, much more ozone is produced.
Recently, an organic photosensitive body for the use of a minus discharge is widely employed as a photosensitive body. Also, the environmental standard for generation of various kinds of gases including the ozone gas became severer. Thus, a countermeasure is keenly demanded. The corona discharge system is further encountered with the problem that nitrogen compounds (NOx), etc., which are by-produced by ozone, are attracted to the surface of the photosensitive body to cause an abnormal image.
In view of the above, there is developed a contact-to-charge type image forming apparatus as a substitute of the corona discharge system. This new type of an image forming apparatus employs a charging member such as a charging roller to be contacted with the surface of the photosensitive body. According to this contact-to-charge system, the charging roller supplied with voltage is brought into contact with the surface of the photosensitive body. The surface of the photosensitive body is uniformly charged by electric discharge through gaps which, in the strict sense of the word, exist between the charging roller and the photosensitive body. Therefore, in the new system, the voltage to be supplied to the surface of the photosensitive body can be lowered compared with that of the corona discharge system. Thus, the contact-to-charge system has the advantage that the amount of ozone to be produced is radically reduced.
However, the contact-to-charge system also has the shortcomings that a required electric potential to be charged is difficult to be maintained at a certain level and a non-uniform charging occasionally occurs. The reason is that when the circumstance of use is changed, electrical characteristics of the charging roller, such as a value of resistance and a dielectric constant, are changed by that.
As means for preventing the non-uniform charging which is caused, as mentioned, by the change of circumstance of use, the following techniques are known.
In an official gazette of Japanese Laid-Open Patent Application No. Hei 4-186381, there is disclosed a technique for detecting the temperature of a charging roller using a sensor and changing the voltage to be supplied to the charging roller in accordance with the detected temperature. For example, in a case that the charging roller is used under a high-temperature circumstance, an alternating current (AC) is supplied to the charging roller so that there can be obtained a peak inter-voltage as large as more than two times the voltage of the starting time when a direct current (DC) is supplied to the charging roller. On the other hand, in a case that the charging roller is used under a low temperature circumstance, a component of an alternating current to be supplied to the charging roller is increased in accordance with the temperature detected by the sensor (see FIG. 3 of the official gazette).
Also, there is disclosed a similar technique in an official gazette of Japanese Laid-Open Patent Application No. Hei 4-316064, in which the temperature of a charging roller is measured by temperature measuring means so that the alternating current voltage to be supplied to the charging roller can be changed based on the measured temperature. According to the technique disclosed in this official gazette, an alternating current voltage preliminarily combined with a direct current voltage is supplied to the charging roller. It is programmed such that a value of resistance and a dielectric constant of the charging roller are found based on information obtained by the temperature measuring means and the alternating current voltage is varied based on a preliminarily obtained test data.
However, the techniques disclosed in the official gazettes of Japanese Laid-Open Patent Application Nos. Hei 4-186381 and Hei 4-316064 have the following problems. When an alternating current voltage is supplied to the charging roller, a vibration sound is generated from the charging roller. On the other hand, when a direct current voltage is supplied to the charging roller in accordance with the change of temperature of the circumstance, the relations between the supplying voltages by temperatures of circumstance and the surface electric potential (charged electric potential) are changed in inclination and position as indicated by H, H' and H" in the graph of FIG. 7.
In another official gazette of Japanese Laid-Open Patent Application No. Hei 6-35302, there is disclosed a technique for controlling the voltage to be supplied to a charging roller in accordance with a layer thickness of a photosensitive body and circumstance moisture. For example, when the layer thickness of the photosensitive body is reduced due to the increased number of times for forming an image thereon, a voltage/current characteristic corresponding to the capacity with respect to the thickness of the photosensitive body at that time is detected and a corrected optimum supplying voltage is supplied to the charging roller based on the detection. That is, as the layer thickness is reduced, an amount of detected current is increased when a constant voltage is supplied to the non-image forming portion. A voltage reducing correction is applied to the voltage value to be supplied to the non-image forming portion in accordance with the increased amount, so that the charging roller is prevented from being overly charged. Also, when the value of resistance is increased due to fluctuation of the circumstance moisture in the layer of resistance on the charging roller, the amount of detected current is reduced, thus obviating shortage of charging which would otherwise require a voltage increasing correction applied to the voltage value to be supplied to the non-image forming portion.
Such a technique for correcting the supplying voltage to the charging roller in accordance with the layer thickness of the photosensitive body is also disclosed in the official gazette of the Japanese Laid-Open Patent Application No. Hei 5-27557. According to the technique disclosed in this official gazette, any diminution of the layer caused by the increased number of copies is detected by a copy counter and the voltage to be supplied to the charge roller is lowered in accordance with the diminution of the layer, thereby maintaining a constant surface electric potential of the photosensitive body.
However, when a charging roller is used as a charging member, there arises a problem that because the charging ability is varied depending on conditions of the circumstance, any change in surface potential (charging potential) with respect to the thickness of the photosensitive layer on the photosensitive body is different depending on temperature of the charging roller (FIG. 8) and a constant charging potential maintained by a uniform correction.
On the other hand, of all the surface of the photosensitive body, there is a portion corresponding to an area appeared between a first transfer paper and a second transfer paper in a circumferential direction of the photosensitive body, i.e., a no-image forming area (this area is not always fixed or constant with respect to the circumferential direction of the photosensitive body), which portion or area is not contacted with the transfer paper and therefore, no image is formed on that portion or area.
In a copying machine of a recent year, therefore, there is employed a technique for controlling an amount of toner to actually adhere to a transfer paper by detecting an amount (concentration) of toner temporarily attracted to the no-image forming area.
In such an image forming apparatus, referred to as a two-component type copying machine, when an image forming process for a predetermined number of transfer papers is finished, the surface of the photosensitive body is caused to a predetermined voltage by a charging roller and then exposed by an exposure device in order to form a concentration-control pattern in the no-image forming area. Then, a visible image is formed by a developing device. An amount of toner attracted to the no-image forming area is detected by a photo sensor or the like. Based on the result of detection, the supply of toner from a toner supplying device to the developing device is controlled to a preset value.
At that time, the surface electric potential of the no-image forming area is different from that of the image forming portion or area. As a consequence, a plurality of electric potentials (target values) are set with respect to a single photosensitive body.
Incidentally, among the above-mentioned official gazettes, FIG. 10 of the official gazette of the Japanese Laid-Open Patent Application No. Hei 6-35302 discloses one example of such a technique for correcting a supplying voltage taking into consideration a reference value with respect to a plurality of target values.
According to the teaching of the correcting method disclosed in the above-mentioned official gazette, an amount of correction with respect to a reference supplying voltage is equal (constant) as apparent only from the fact that a side line in a graph of FIG. 10 is moved in a parallel relation. This is apparently based on an idea that when the thickness of the layer on the photosensitive body is reduced, the voltage at the start of discharge is also reduced or lowered, and therefore electric charge required is increased.
For example, presume that the reference thickness of layer is represented by a and that there are two different target electric potentials A and B. A correction is made in accordance with a correction rule that when the current thickness of the layer is varied to b at the time of the target electric potential A, a reference supplying voltage corresponding to the target electric potential A is added with .alpha.. Similarly, a correction is also made in accordance with a correction rule that when the current thickness of the layer is varied to b at the time of the target electric potential B, a reference supplying voltage corresponding to the target electric potential B is added with .alpha.. That is, the correction value is always "+.alpha." whether the target electric potential is A or B.
Also, a correction necessitated by temperature change is made also based on a simple idea, for example, that if the temperature of the charging roller is low, resistance is large and the surface electric potential of the charging roller is decreased. As a consequence, it is customary to think that an amount of correction with respect to the reference supplying voltage is equal irrespective of the target electric potential.
For example, presume that the reference temperature is represented by a and that there are different electric potentials A and B. A correction is made in accordance with a correction rule that when the current temperature is varied to b at the time of the target electric potential A, a reference supplying voltage corresponding to the target electric potential A is added with .alpha.. Similarly, a correction is also made in accordance with a correction rule that when the current temperature is varied to b at the time of the target electric potential B, a reference supplying voltage corresponding to the target electric potential B is added with .alpha.. That is, the correction value is always "+.alpha." whether the target electric potential is A or B.
However, when such a uniform correction is made, there is encountered a problem that controllability of electric potential with respect to one of the target values is remarkably reduced. As a consequence, an uneven electric potential occurs in the no-image forming area. The results are that the sensor makes a wrong detection and an incorrect amount of toner is supplied, thus causing an unstable image quality.